This invention relates to electronic communication systems and more particularly to communication devices in mobile radio communication systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Third generation (3G) mobile radio communication systems, such as the universal mobile telecommunications system (UMTS), are digital communication systems that are standardized by the Third Generation Partnership Project (3GPP). The UMTS employs WCDMA for the air interfaces between user equipments (UEs) and NodeBs, or base stations, in the system. This application focuses on a UMTS system for economy of explanation, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
FIG. 1 depicts a mobile radio communication system 100, which may be, for example, a UMTS. Radio network controllers (RNCs) 110, 112, 114 control various radio network functions, including for example radio access bearer setup, diversity handover, etc., for communicating with UEs. More generally, each RNC directs calls from a UE, such as a mobile phone, via one or more appropriate NodeBs 120, 122, 124, which communicate with each other through downlink (i.e., NodeB-to-UE or forward) and uplink (i.e., UE-to-NodeB or reverse) channels.
In FIG. 1, each RNC 110, 112, 114 is shown coupled to a respective NodeB 120, 122, 124, but this is not necessary; in general, an RNC can be coupled to plural NodeBs. The NodeBs are coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. Each NodeB serves a respective geographical area that can be divided into one or more cell(s). FIG. 1 depicts NodeB 120 serving a Cell A, NodeB 122 serving a Cell B, and NodeB 124 serving a Cell C, with the cells non-overlapping, but this is also not necessary.
In general, the RNCs are connected with external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes like a mobile switching center and/or a general packet radio service (GPRS) node. In FIG. 1, the RNC 110 is shown connected to a Serving GPRS Support Node (SGSN) 130, and the RNCs 112, 114 are connected to an SGSN 132. The SGSNs 130, 132 are connected to Gateway GPRS Support Node (GGSN) 140.
A multimedia broadcast and multicast service (MBMS) for the frequency division duplex (FDD) aspect of a UMTS is standardized by 3GPP. MBMS is described in, for example, 3GPP Technical Specification (TS) 23.246 ver. 6.12.0, Multimedia Broadcast/Multicast Service (MBMS); Architecture and Functional Description (Release 6) (June 2007), among other places. MBMS provides both point-to-point (PTP) and point-to-multipoint (PTM) multimedia services. In PTM, the same data (e.g., text, audio, pictures, video) is transmitted from a single source to multiple receivers.
In the example of a network topology depicted in FIG. 1, a Broadcast/Multicast Service Center (BM-SC) 150 serves as entry point for MBMS services. Streams of data for the various services from the BM-SC 150 go through the GGSN 140 and SGSNs 130, 132 to the RNCs 110, 112, 114. Each RNC decides whether a service is to be broadcast to multiple UEs in a cell (PTM) or sent to one or a small number of UEs (PTP) in a cell based on either a static configuration or on the number of UEs interested in the service. The number can be derived by a procedure called counting. FIG. 1 depicts Service X being broadcast by NodeB 120 to multiple UEs in Cell A, Services X and Y being sent by the NodeB 122 to respective UEs in Cell B, and Service Y being sent by the NodeB 124 to a UE in Cell C.
MBMS enables high-speed and high-quality broadcast, or multicast, transmission to UEs. To enhance the quality and bit rate of MBMS transmission, multicast on Layer 1 is used, i.e., a UE should be able to receive multiple replicas of the same bitstream from different NodeBs. UEs also should be able to receive bitstreams of 128 kilobits per second (kbps) and 256 kbps and to do selective combining on the radio link control (RLC) level (Layer 2). Thus, a UE separately processes different bitstreams on Layer 1, and the UE's RLC entity selects transport blocks, called RLC protocol data units (PDUs), from the different streams based on whether they pass a cyclic redundancy check (CRC) or not. Instead of using RLC PDUs, selective combining and soft combining can also be carried out between multiple antennas, polarization angles, etc. Soft combining is typically performed on Layer 1.
In an MBMS-enabled UMTS, a NodeB broadcasts control information, such as what services are currently available, in which mode (PTM or PTP) a service is available, and other configuration information, on a logical channel called a multicast control channel (MCCH). The same MCCH information is repeated during each of successive repetition periods within each of successive modification periods, and actual user data is carried on a logical channel called an MBMS traffic channel (MTCH).
FIG. 2 depicts two successive modification periods of an MCCH broadcast by a NodeB, with time on the horizontal axis in the figure. As described above, the same MCCH content is repeated in each of the repetition periods during a modification period. It will be understood that FIG. 2 shows only an example of the particular numbers of repetition periods within a modification period.
It is important to note that although a UE can simultaneously read data from several cells (i.e., a UE can simultaneously receive several MTCHs carrying the same data and combine the received streams), the MCCH is cell-specific and so a UE can read only one MCCH at a time. In this application, the cell having an MCCH that the UE has chosen to read is called the UE's “MBMS serving cell”.
Choosing an MBMS serving cell while complying with applicable 3GPP specifications is reasonably straightforward for non-dedicated UE states, i.e., IDLE, CELL_PCH, URA_PCH, and CELL_FACH. For example, the UE can simply choose as its MBMS serving cell the cell on which the UE camps in non-dedicated UE states. That camped-on cell is the UE's “serving cell”, and procedures for selecting and re-selecting the serving cell are defined in, for example, Sections 5.2 and 5.4 of 3GPP TS 25.304 V6.10.0, UE Procedures in Idle Mode and Procedures for Cell Reselection in Connected Mode (Release 6) (March 2008) and described in U.S. Patent Application Publication No. US 2008/0031368 A1 by Lindoff et al. for “Efficient Cell Selection”. Such procedures include both time and signal-quality hysteresis to avoid problems of “ping-ponging” between serving cells, as described for example in U.S. Patent Application Publication No. US 2009/0059871 A1 by Nader et al. for “Time-to-Trigger Handling Methods and Apparatus”.
PTP MBMS as well as voice/data calls and other communications use the dedicated UE state CELL_DCH rather than a non-dedicated state like IDLE. In the CELL_DCH state, the UE is connected to a set of plural cells that is called the Active Set, and so a UE does not have a typical serving cell in its CELL_DCH state. Thus, choosing an MBMS serving cell in the CELL_DCH state is not straightforward and there is a need for a method for choosing an MBMS serving cell in a stable way, which is to say, without ping-ponging and other problems. It will be appreciated that this problem arises from the UE's need to read a cell-specific MCCH; the UE can combine MTCHs from several cells just as it does with signals from the Active Set for normal non-MBMS activity.